According to nTLDStats.com, it looks like there are now more than 100,000 .Dev domain names registered. I can see more than 102,000 .Dev domain names in the zone file, and the number continues to grow.
The .Dev domain name extension is managed by Google, and .Dev domain names became available to register via the early access program on February 19. Since general availability phase began on February 28, the number of registrations has grown strongly. From what I can tell, it looks like there were about 5,000 domain names registered via the EAP, and in the last week and a half, over 95,000 domain names have been registered.